In recent years automobile manufacturers have offered as optional equipment rear windows which can be defrosted and/or defogged by use of an electrically conductive grid permanently attached to the window. In order to defrost quickly, the circuit must be capable of supplying large amounts of power from a low voltage power source, for example, 12 volts. Furthermore, the lines of the conductive grid must be sufficiently narrow in order to maintain visibility through the rear window.
Heretofore, the materials used for the preparation of window defogging grids have mostly been thick film silver conductors which are prepared from paste comprising finely divided silver powder particles and glass frit dispersed in an organic medium. In a typical application a paste containing by weight 70% silver powder, 5% glass frit and 25% organic medium is screen printed through a 180 Standard Mesh Screen onto a flat, unformed glass rear window. The printed composition is dried for at least 2 minutes at about 150.degree. C. and the entire element is then fired in air for from 2 to 5 minutes at 650.degree. C. After firing, the softened glass is shaped by pressing into a mold and then tempered by rapidly cooling. During the firing cycle the organic medium is removed by evaporation and pyrolysis. The glass and silver are sintered to form a continuous conductive path with the glass acting as binder.
The silver compositions currently used yield upon firing resistances of from 2 to 15 milliohms per square. The resistance requirements vary according to the size of the window and hence the conductive grid. Conductors for large window areas need more electrical current because they have more area to defrost and therefore have much lower resistance requirements. Thus, the larger rear window area is typical of full sized cars require as little as 2 milliohms per square resistance, whereas the relatively small rear window area which is typical of compact cars can utilize compositions having resistances of as high as 15 milliohms per square.
Because of the current trend toward smaller cars the automotive industry anticipates a decline in the need for very low resistance silver compositions (2 to 4 milliohms per square) and forecasts suggest that the future resistance requirements will be for compositions of from 3 to 8 milliohms per square.
Such resistance requirements for defoggers are easily met by noble metal conductors, particularly silver, which is currently the most widely used conductor material.
In the manufacture of automotive defoggers, an important criterion has been the appearance of the interface between the glass and conductor pattern--particularly color. By far the most predominant type of glass used in the manufacture of automotive windows is soda-lime glass made by the float glass process in which the molten soda-lime glass is cast upon a long pool of molten tin or tin alloy in a controlled atmosphere. This process produces a glass of nearly perfect flatness and excellent thickness uniformity without the necessity of grinding and polishing. A typical soda-lime glass is comprised by weight of 72.5% SiO.sub.2, 15.9% Na.sub.2 O, 6.5% CaO, 3.0% MgO, 1.3% Al.sub.2 O.sub.3, 0.3% K.sub.2 O and 0.3% other oxides and impurities.
When silver is used as a conductive material, a naturally dark brown color is produced at the glass-conductor interface even without the addition of colorants when the conductor is printed on the "tin side" of the glass, i.e., on the side of the glass which was next to the tin in the float process. However, when the conductor is printed on the "air side" of the glass, i.e., on the side exposed to the atmosphere, no such color development takes place. The reason for this difference between the "air" and "tin" sides is not known with certainty but is believed to be due to insolubilization of silver at the interface by reaction with minute amounts of residual tin from the float process.
In the manufacturing process it is, however, preferred to print the conductor on the air side in order to avoid certain handling difficulties. For this reason formulations of thick film pastes for this use have contained various coloring agents so that they may be printed on the air side of the glass with suitable color development.
Heretofore the colorants for soda-lime glass have been silver salts such as Ag.sub.2 SO.sub.4 and Ag.sub.2 S which upon firing take part in ion exchange reactions with components of the glass substrate to form particles of silver at the interface between the substrate glass and the conductor composition printed thereon. These particles then form colored agglomerates. This phenomenon is believed to take place by the following sequence of reactions: EQU (Ag.sup.+1).sub.paste +(Na.sup.+).sub.glass .fwdarw.(Ag.sup.+1).sub.glass +(Na.sup.+).sub.paste residue ( 1) EQU (Ag.sup.+).sub.glass +(Fe.sup.2+).sub.glass .fwdarw.(Ag.sup.o).sub.glass +(Fe.sup.3+).sub.glass ( 2)
It should be noted here that gold and copper act in the same manner as silver and therefore could be used in the same way as silver. However, they are not used for this purpose to any substantial extent.
In clear glass substrates, Fe.sup.2+ ions are present in small amounts as an impurity while in tinted glass more substantial amounts are present as a coloring additive to the glass. Thus, the above-postulated sequence of reactions is possible whether or not iron is specifically added to the substrate.
It is usual in the manufacture of automotive windows having defogger circuits printed thereon to store them for a time in the outdoors after they have been fired and cooled. Often, after some prolonged period of storage, they are taken inside to a warm area where they are subject to condensation of atmospheric moisture thereon. It is has been found that condensation of this type is of sufficient magnitude that the sodium ions in the paste residue [See Equation (1)] react with sulfate or sulfide ions from the silver salt colorant; and, as the moisture evaporates a white residue is formed on the window, in areas near the printed conductor pattern. (If a copper salt colorant were used, the same phenomenon could take place by analogous reactions). Because of the unsightly nature of these deposits, manufacturers of automotive windows with defogger circuits printed therein have sought to overcome this. However, neither alternative colorants nor ways of suppressing the reactions have been forthcoming before now.